Could you please explain meaning of the phrase generally and meaning of "wind" in this case.
My back struck iron: the trailer’s wall. My feet snapped over my head and I continued my graceless plunge to the ground. The first fall was seven or eight feet, the second perhaps ten. I was relieved to taste dirt. I lay on my back for perhaps fifteen seconds before the engine growled to silence and I heard Dad’s heavy step. “What happened?” he said, kneeling next to me. “I fell out,” I wheezed. The wind had been knocked out of me, and there was a powerful throbbing in my back, as if I’d been cut in two.